The total volume of Heavy Duty Shipping Sacks (HDSS) is distributed among the following three materials: paper, polyethylene and polypropylene woven. The main advantage of bags formed from polypropylene is their superior toughness, as compared to polyethylene bags or paper bags. For this reason, the majority of the polypropylene bags are used for the packaging of heavy consumer goods, such as building and construction materials, fertilizers, and pet foods. In the chemical industry, polyethylene bags are mostly used due to their many advantages, such as good barrier to moisture, good seal ability and good printing quality.
The introduction of form fill and seal systems (FFS) has been a key factor in the conversion from paper to polyethylene bags. In the FFS process, the bag is formed (sealed vertically and sealed at bottom of bag) on the bagging machine from a roll of film, and immediately the formed bag is filled and sealed across the top. Thus, the bagging process had been completely integrated into a high speed filling process.
Polypropylene bags are desirable to use in a FFS process for the packaging of heavy powders and fillers, such as fertilizers and pet foods. However, such bags have inferior sealing properties and barrier properties, as compared to polyethylene bags. Conventional solutions to improve impermeability of woven polypropylene bags involve applying a coating onto a polypropylene substrate. But, problems still remain for the use of polypropylene substrates in FFS processes due to poor sealing performance at required sealing temperatures. For this reason, the current woven polypropylene bags remain restricted to pre-made bags which are sealed by stitches or patches.
U.S. Pat. No. 4,844,958 discloses a circular woven fabric having a woven tubular body with thermoplastic tapes secured thereto, and an extrusion polymeric coating fuse bonded to the tapes and the woven fabric. The extrusion coated circular woven fabric is useful in intermediate bulk containers for carrying particulates. Polypropylenes, polyethylenes, LLDPE, LDPE and blends thereof may be used for the extrusion coating.
U.K. Patent Specification No. 1265755 discloses a laminated web consisting of a woven layer and a continuous thermoplastic coating thereon. The woven layer is made by interweaving tapes, made from a crystalline polyolefin, and the coating contains at least ten percent, by weight, of the same crystalline olefin polymer and at least 40 percent, by weight, of low density polyethylene.
JP2001030446A (abstract) discloses a woven fabric base material consisting of a thermoplastic resin drawn thread that is coat-treated, and laminated with a resin layer comprising a polyethylene resin material, containing an ethylene-α-olefin copolymer that has a density of 0.870-0.920 g/cc, a MFR of 1-100 g/10 min, a maximum peak temperature of a TREF elution curve of 25-85° C., and an amount elution at 90° C. or lower of at least 90 weight percent.
International Publication No. WO 2009/091952 discloses a perforated coated substrate comprising at least the following: i) a first layer, ii) a second layer, and iii) a woven and/or nonwoven web; and wherein the second layer has a lower softening and/or melting temperature, as compared with the respective softening and/or melting temperatures of the first layer, and the respective softening and/or melting temperatures of the web. The layers of the coated substrate have perforations with a common center.
International Publication No. WO 2008/100720 discloses a composition suitable for use in extrusion coating processes to form a heat sealable film. The composition comprises from 50 to 92 percent, by weight, of a propylene based plastomer or elastomer, and from 8 to 50 percent, by weight, of low density polyethylene. The invention also relates to heat sealable seals, and particularly peelable seals made from the compositions.
U.S. Pat. No. 6,387,491 discloses a flap fusion sheet, formed by fusing a flap, continuous in the longitudinal direction, at least at positions other than end portions in the width direction, at one side of a continuous backing sheet. The backing sheet is formed by laminating layers made of a low melting point polyolefin on both sides of a flat yarn cloth made of high melting point polyolefin.
U.S. Publication No. 2007/0254176 discloses an article comprising at least two layers, a first or low crystallinity layer, comprising a low crystallinity polymer, and a second or high crystallinity layer, comprising a high crystallinity polymer. The high crystallinity polymer has a melting point, as determined by Differential Scanning calorimetry (DSC), that is about the same or within less than 25° C. of the melting point of the low crystallinity polymer. The article is elongated at a temperature below the melting point of the low crystallinity polymer, in at least one direction, to an elongation of at least about 50% of its original length or width, to form a pre-stretched article. Preferably, the high crystallinity layer is capable of undergoing plastic deformation upon the elongation.
International Publication No. WO 2009/033196 (abstract) discloses a coated fabric that comprises a fabric from a monoaxially drawn polymer tape, especially polyolefin or polyester tape, preferably polypropylene or polyethylene terephthalate tape. The fabric is coated with a sealing layer, produced of a thermoplastic synthetic with a melting point below the crystallite melting point of the fabric tape material. The coated fabric is especially suitable for hot air, ultrasonic, heated tool, infrared, or laser beam welding techniques.
Additional coated substrates, films, and/or bags are described in U.S. Publication Nos. 2006/0172143 and 2006/0204696, International Publication Nos. WO 2007/05059, WO 2007/008753, WO 2010/002837, U.K. Patent Application No. 2039788A, and Japanese Patent Application No. JP59011250A (abstract).
There remains a need for coated substrates with improved seal strengths for use in FFS processes. There is a further need for such bags that have sufficient strength to hold heavy powders and fillers. These needs have been met by the following invention.